When I first got into AR- 15's, I couldn't quite fathom how the exact same product was made by so many different manufacturers.

If you liken an AR-15 to a vehicle, it would be similar to Ford, Chevy, Dodge, et al, making a vehicle called the "Car-15" without any patent infringements. Only slight nuances occurring between each make.

In the world of AR's however, it is not the case. Do any AR-15 manufacturers pay royalties to anyone regarding the general design of the rifle?